1. Field of the Invention
The present invention relates to a gas flow-rate measuring apparatus which is preferable in use for detecting an intake-air flow-rate of a vehicular engine.
2. Description of the Related Art
Generally a vehicular engine of a fuel injection type is provided with an apparatus for accurately measuring a flow-rate of intake air in order to derive the amount of fuel injection with high accuracy. Japanese Patent Provisional Publication No. 61-65053 discloses a gas flow-rate measuring apparatus which is constituted by a pipe body, a housing including a bypass passage and a flow-rate detection element. Gas to be detected by this apparatus is led to the pipe body to which the housing is installed. The housing is provided with the bypass passage having a circular passage cross-section so that part of the gas flows through the bypass passage. The flow-rate detection element of a bobbin-type hot-wire probe is installed in the bypass passage. A passage length of the bypass passage is formed longer than that of a main passage of the pipe body in order to reduce the influence of the pulsating flow caused by the engine operation. More particularly, the influence of the pulsating flow is suppressed by generating a pressure difference at the bypass passage with respect to a main passage. Further, the flow-rate detection element is disposed at a passage restricting portion in the bypass passage.
However, the pulsating flow of the intake air largely affects the passage restricting portion of the bypass passage such that the detection error of the hot-wire type flow-rate detection element becomes large. Such error due to the pulsating flow is increased according to the increase of the magnitude of the pulsating flow, as show by a dotted line in FIG. 13.